To control the fuel supply to an engine, a fuel controller is disclosed for example Tokkai Hei 3-222849 published by the Japanese Patent Office wherein an air flow meter is installed upstream of an intake air throttle, and a fuel quantity is calculated based on an intake air flowrate measured by the air flow meter.
This controller comprises a control unit consisting of a microprocessor, the output of the air flow meter (voltage) being converted to a digital value which is then input to the control unit.
However, as there is a slight response delay from the real change of air flowrate at the air flow meter to this input signal, signal processing is performed in the control unit to compensate for this response delay.
Moreover, as there is an overshoot in the computed flowrate Qs due to the pressure change in the air intake pipe during rapid acceleration, a basic fuel injection pulse width Avtp corresponding to the cylinder intake air volume is calculated in the control unit by means of the following equation, and this calculated value is output to a fuel injection valve installed in the intake port of the engine: EQU Tp=(Qs/N)*K EQU Avtp=Tp*Fload+Avtp.sub.n-1 *(1-Fload)
where:
N=engine speed PA1 K=constant PA1 Fload=air intake pipe delay coefficient PA1 Avtp.sub.n-1 =Avtp on Immediately preceding occasion
However, in engines with less than six cylinders, if there is a very large air pulsation when the air intake throttle is fully open, the computed result for the intake air volume may be a negative value due to backflow of air accompanying the pulsation. Further, the computed result may be extremely large during rapid acceleration.
The flowrate applied in actual fuel control is therefore limited to within a predetermined range, for example from 0 to an upper limit of a preset range. This is due to the fact that the required precision in air-fuel ratio control was conventionally not so high, and to the fact that the amount of data increased too much when the preset range was widened.
However, if the above calculation is performed using an upper limit or lower limit for the air intake volume, the effect of the difference or error between the real value and the value of the upper or lower limit on the calculation of fuel injection amount does not terminate after one injection. In other words, as the calculation of fuel amount takes the value on the immediately preceding occasion into account, the error is perpetuated over a long time period as it gradually decreases.
FIG. 34 is a graphical representation of the change of intake air volume, Avtpr and air-fuel ratio during rapid acceleration. If the above difference or error is clipped as described hereintofore, the air-fuel ratio at the start of acceleration shifts to lean as shown by the dotted line in the figure. On the other hand, in the latter half of the graph where the intake air volume becomes a negative value due to pulsation, the negative part is clipped, so the air-fuel ratio tends to rich.
This clipping applies not only to the intake air volume measured by the air flow meter, but also to the opening signal supplied to the air intake throttle or a flow control valve in a supplementary air passage, or to the calculation of fuel injection amount. At the present time, severe requirements are imposed on the composition of exhaust gas, and the effect of this clipping on the air-fuel ratio therefore cannot be ignored.